1. Field of the Invention
The present invention relates to a process for producing highly pure 2,6-dimethylnaphthalene. More particularly, it pertains to a process for efficiently producing, in high yield in an industrially advantageous manner, highly pure 2,6-dimethylnaphthalene useful as a starting raw material for a 2,6-naphthalenedicarboxylic acid from the mixture of dimethylnaphthalenes comprising as a principal component, 1,5-dimethylnaphthalene obtained preferably from o-xylene and butadiene as starting raw materials.
2. Description of the Related Arts
It has heretofore been known that a 2,6-naphthalenedicarboxylic acid and an ester thereof are each a compound of industrial importance as a starting raw material for a high performance polyester which is employed for the production of polyethylene naphthalate in the form of fiber, film and the like that is excellent in tensile strength and heat resistance.
The 2,6-naphthalenedicarboxylic acid and an ester thereof that are used for such a purpose are required to be highly pure and besides 2,6-dimethylnaphthalene (hereinafter dimethylnaphthalene is sometimes abbreviated to "DMN") which is used as a starting raw material thereof is required to be also highly pure because of the reasons described hereunder.
Specifically, 2,6-DMN, when being low in purity, causes the impurities contained therein to be oxidized or esterified, and eventually lowers the purity of a 2,6-naphthalenedicarboxylic acid and an ester thereof when being produced therefrom. A part of the impurities that are formed during the steps of oxidation and esterification, originating from the impurities in 2,6-DMN is extremely difficult to remove, thereby making it also extremely difficult to obtain a 2,6-naphthalenedicarboxylic acid or a dimethyl 2,6-naphthalenedicarboxylate each having high purity. In addition, such impurities, when being present in 2,6-DMN, deteriorates not only the purity of the above-mentioned acid and ester, but also the yield thereof based on 2,6-DMN to a remarkable extent. It is therefore, indispensable that highly pure 2,6-DMN be obtained in order to produce a 2,6-naphthalenedicarboxylic acid and a dimethyl 2,6-naphthalenedicarboxylate under industrially advantageous conditions. DMN has 10 isomers according to the positions of two methyl groups. Accordingly, 2,6-DMN as a starting raw material for a 2,6-naphthalenedicarboxylic acid is required to be a highly pure product substantially free from any of the isomers other than 2,6-DMN.
As the process for producing 2,6-DMN, there are available for example, a process in which 2,6-DMN is isolated from a tar fraction or a petroleum fraction, a process in which naphthalene or methylnaphthalene is methylated, succeedingly isomerized and separated and the like processes. Since the fractions and isomerization reaction products contain almost all of the 10 kinds of isomers, 2,6-DMN needs to be isolated from the mixture of a lot of isomers.
On the other hand, Japanese Patent Application Laid-Open Nos. 134634/1974, 8935/1975, 76852/1973 and 129534/1975 disclose a process for producing o-tolylpentene-2 in high yield from o-xylene and butadiene; a process for producing 1,5-dimethyltetralin by cyclizing o-tolylpentene-2; a process for producing 1,5-DMN in high yield and in high selectivity by dehydrogenating 1,5-dimethyltetralin; and a process for producing a mixture of isomers consisting essentially of 1,5-, 1,6- and 2,6-DMNs by isomerizing 1,5-DMN. Accordingly, by combining the above-mentioned processes it is made possible to produce a mixture of isomers consisting essentially of 1,5-, 1,6- and 2,6-DMNs from o-xylene and butadiene. Thus, there is made available a process for producing 2,6-DMN by isolating 2,6-DMN from the aforesaid mixture.
As described hereinbefore, any of the processes for producing 2,6-DMN that have heretofore been available makes it necessary to isolate 2,6-DMN from the mixture of isomers to recovery the same. However, it is extremely difficult to purify 2,6-DMN by means of distillation which is frequently applied to the separation and purification of ordinary organic compounds, since 10 kinds of isomers have each a boiling point very close to one another as shown hereunder together with the melting point.
______________________________________ Boiling point (.degree. C.) Melting point (.degree. C.) ______________________________________ 1,5-DMN 269 82 1,6-DMN 266 -16 2,6-DMN 262 112 1,7-DMN 263 -14 1,8-DMN 270 65 2,7-DMN 262 98 1,3-DMN 265 -4.2 1,4-DMN 265 6 2,3-DMN 269 104 1,2-DMN 271 -3.5 ______________________________________
As is clear from the table, 2,6-DMN has a highest melting point of all the DMN isomers. On the other hand, it is known that 2,6-DMN forms a eutectic together with at least one of 1,5-DMN, 2,7-DMN and 2,3-DMN. It is therefore, necessary that the ratio by amount of 2,6-DMN in the isomer mixture to the isomers be more than the compositional ratio of the eutectic in order to precipitate 2,6-DMN as a crystal by means of crystallization from the mixture of the isomers. That is to say, the condition under which 2,6-DMN is at first precipitated as a crystal by cooling is that the molar ratios of 1,5-DMN, 2,7-DMN and 2,3-DMN each in the mixture of the isomers to 2,6-DMN in the same are not more than 1.9, 1.4 and 1.1, respectively.
As a method for isolating 2,6-DMN from the mixture of the isomers, there are proposed a crystallization method, an adsorption method, a method in which 2,6-DMN is caused to form a complex by the use of a certain kind of an organic compound, the resultant complex is separated and then it is decomposed to recover 2,6-DMN, and the like methods. Of these methods, crystallization method is most simple, convenient and suitable as an industrial isolation method.
In particular, in the case of producing a mixture of isomers consisting essentially of 1,5-, 1,6- and 2,6-DMN from o-xylene and butadiene and isolating 2,6-DMN therefrom, a crystallization method is effective because of the comparatively small number of isomers in the starting raw material to be purified. In the case of methylating naphthalenes, isomerizing the reaction product and isolating 2,6-DMN or in the case of isolating the same from a tar fraction or a petroleum fraction, the combination of an adsorption method and a crystallization method is usually employed, since 2,6-DMN needs to be isolated from the mixture of a large number of isomers.
In the case of recovering 2,6-DMN by separating it from a mixture of various DMN isomers typified by the mixture of DMN isomers comprising 1,5-, 1,6- and 2,6-DMN as the principal components which mixture is obtained from o-xylene and butadiene, isomerization is necessary in order to enhance the yield of 2,6-DMN. Specifically it is effective to enhance the yield of 2,6-DMN by repeating the steps of isomerizing the mixture of DMN isomers obtained as the mother liquor separated from precipitated 2,6-DMN to enhance 2,6-DMN concentration; and recovering 2,6-DMN by separating it from the resultant mixture of DMN isomers.
It is well known that in the case of isomerizing DMN, isomerization between adjacent .beta.-positions and isomerization of methyl-migration from one ring to another are unlikely to take place as compared with that between .alpha.-position and .beta.-position. Specifically, the above-mentioned 10 DMN isomers are classified into four groups, namely A to D groups as undermentioned with regard to isomerization, and isomerization among different groups is unlikely to take place as compared with that in the same group.
Group A - - - 1,5-DMN, 1,6-DMN, 2,6-DMN PA1 Group B - - - 1,8-DMN, 1,7-DMN, 2,7-DMN PA1 Group C - - - 1,4-DMN, 1,3-DMN, 2,3-DMN PA1 Group D - - - 1,2-DMN
There is disclosed in Japanese Patent Publication No. 24331/1982 (U.S. Pat. No. 3,957,896) a process for producing 2,6-DMN by utilizing the fact that the use of a specific isomerization catalyst causes isomerization to a different group which process comprises isomerizing, in the presence of such a catalyst, a mixture of DMN which contains to some extent, DMN not belonging to 2,6-DMN group as a starting raw material, crystallizing the resultant isomerization liquid product, separating 2,6-DMN therefrom and recycling for reuse the mother liquor separated from 2,6-DMN. However, crystallization was carried out according to the above-mentioned process by the present inventors by the use of the mixture of isomers which was obtained through the isomerization of DMN containing to some extent, DMN not belonging to 2,6-DMN group with the result that highly pure 2,6-DMN crystal was proved to be unobtainable.